U.S. Pat. Nos. 5,041,748, 2,918,827, and 2,446,393 show linear screw actuators where a rotatable input member is driven by an electric motor, and the output member is held against rotation relative to the input member so that a screw thread connection between the input and output members causes the output member to translate axially as the axially fixed input member is rotated by the electric motor. In each of the actuators the axially translatable output member is held against rotation by its connection to the device which is actuated by the actuator. Such an arrangement does not lend itself to a high speed/high load actuator and it is an object of the present invention to provide an actuator which is robust, and capable of rapid actuation in a high load environment.